With regard to Pokémon - Misty, May and Dawn receive insane amounts of fanart in which they are aged up, complete with everything that comes with puberty. Some of the former is the work of the female fanbase - it comes from having a romance subplot with the ChasteIdiot Hero and the need to make her a plausible Love Triangle center in fanfiction, justified in part by how her sisters look...but for the most part it's the guys.
However, Ash, Gary, Drew, Paul and Kenny also have their share of fangirls as well, even Tracey has some. Oddly, the only main character without hoards of fangirls is probably Brock.
Tracey usually does match his description and characterization, though.
Sometimes the Pokemon are drawn as humanoids.
Given how Dragon Ball Z fangirls see him, you probably wouldn't think that Vegeta was a perpetually scowling midget.
This happens to 90% of the characters in Naruto. Hinata is described by the databooks as rather plain and bookish, but fanart tends to make her out to be this gorgeous goddess. Sakura has a small chest and fanart tends to make her out to be a goddess too.
Databook descriptions aside however, Kishimoto draws Hinata buxom and cute as a button while Sakura reaps all the benefits of Generic Cuteness and makes up for her lack of endowment with a different focal point.
Some people are trying to figure out what Shinigami-sama looks like behind his mask. Of course, since his son is the somewhat Bishounen Death The Kid, people (read: mostly fangirls) think that he must be even more good looking.
Well the Hungarian dub DID give him a rather sexy voice when he went serious. On a different note, this may be a moot point since every single time his clothes were damaged, it was quite obviously visible that he had no physical body underneath that shroud.
He's the anthropomorphic personification of death. What do you expect to find under the mask?
The manga has, in fact, indicated that Shinigami might have/had a human form at one point; one, Mosquito knows what the Lines of Sanzu are, which Shinigami doesn't obviously have at the moment, and two Kid is a sort-of clone of Shinigami.
Maka occasionally gets this as well, albeit to a lesser extent; granted, she's fairly cute to begin with, but some fans seemed to miss the memo that she's above an A-cup.
Stein too. He's far from ugly (he's considered attractive in a weird way both In-Universe and out), but a lot of fanart depicts him as a generic Bishounen.
His Shirtless Scene in the anime revealed he's quite well-built. Cue the fangirls drawing him like a bodybuilder.
England sometimes gets hit with a milder version of this trope. While the majority of his fans do recognize that his combative personality and Big Ol' Eyebrows are his major Charm Points and leave them intact, there are still some who "improve" him by softening his personality to make him into an unironic gentleman (even though England being hilariously unsuited for the gentlemanliness he claims to possess in somanyways is one of the defining marks of his character).
The huge lot of Russia fanarts in which he's slimmed down to a generic bishounen build and his nose is suddenly tiny and upturned.
The same can be said of Belarus. Since breast size tends to be genetic and she's Ukraine's sister, it can be justified, but it's rather jarring when fanon "bust charts" portray her as bigger than Hungary, or some fanarts that make her bigger than Ukraine.
Belgium also gets this treatment occasionally, which may be justified as she's usually wearing heavy clothing in canon, but during the 2010 Christmas event, we actually get to see her in her bra, and she has a very average-sized chest.
Although age tends to be vague, the younger-looking Liechtenstein, Seychelles, and Wy get aged up quite often in fanart, sometimes to the point where it's a bit jarring.
America is shown shirtless in canon. While he's no lightweight, his muscles are still about half the size fandom commonly portrays them.
A similar case with Thailand whose body muscle (as shown in Hetalia Bloodbath 2010) is kinda normal, is buffed up in fanart} Then again, these arts often go by the fanon of him being into Muay Thai (the national Thai sport), which requires any practitioner to have rather heavy musculature.
France's body hair can be greatly reduced or absent in fanart. Same with Turkey and Grandpa Rome, but they've got a huge LGBT Fanbase following that love it.
Prussia is sometimes drawn as far more muscular than he is in canon. Sometimes he'll have the same amount of muscle (or even more) as his brother in the same fanart.
North Italy's cuteness will often be changed to handsomeness.
Once in a while, Fandom will raise Latvia's height to around 170 cm.
Ponyo's father, Fujimoto, often loses his long nose and wrinkles in fanart.
Junji Ito's character Souichi has a small but vocal amount of fangirls. You know, the creepy, occult obsessed nail biting anemic boy (and in a dream, psycopathic manchild, who tends to get most of the love)
Kurogane from Tsubasa Reservoir Chronicle. He is genuinely attractive (just very, very masculine) but fangirls tend to portray him as a skinny, feminine Pretty Boy. Some fanartists, however, do the exact opposite and draw him with so much bulk you wonder how his armor still fits.
There is a ton of fanart depicting Renamon with more tits and ass than she normally has; enough to make her the #2 search term on FurAffinity. Renamon is human-sized and has a more adult personality than most of the other Mons in the franchise, so she's shippable... but looks like a Funny Animal at best and canonically has No Biological Sex, which fanartists are more than happy to add.
Kuroko No Basuke gets this a lot, as the basketball players are actually drawn well-muscled (although it comes into play after Art Evolution in the manga), possibly in homage to old school shounen like Slam Dunk along with keeping their physiques realistic; in the flood of art drawn by fangirls on Pixiv and elsewhere, though, they're very much slimmed down, sometimes to the point one is left to question whether one is watching the same show as the fanartist... An interesting counterpoint to this would be in the era of Slam Dunk, when Yaoi Genre doujinshi began to become prominent; possibly due to its newness and the art styles popular at the time, artists stuck to the proportions given to the characters in the anime and manga instead of slimming them down into the Bishounen style that's become ever-more popular today.
The Joker gets this quite frequently, ESPECIALLY after The Dark Knight. Granted, Heath Ledger was a pretty guy, but his Joker looks quite gross with his poor hygiene, wrinkled face and nasty scars. Of course, he becomes a pretty bishonen with cool scars in loads of fanart.
Wolverine. He's a shortnote 5' 3" officially; compare average US height (20+ years old), which is about 5' 9" for males, 5' 4" for females, surly man with more body hair then a bear. Count how many times he's given a height boost, a full-body wax, and just made flat-out generic handsome.
Being played by Hugh Jackman in the movies for one. The comics version hasn't looked like the day-one version in a very long time.
In the Astérix spinoff album Uderzo croqué par ses amis, drawn and written by fans, Asterix gets this treatment in the realistically-drawn stories, which draw him taller, slim and muscular rather than pot-bellied and skinny-limbed, and as a Pretty BoyBarbarian Hero. Some stories give him a larger-than-average nose as concession to his original design's Gag Nose, but others even get rid of that. Inverted in that Obelix usually gets his ugliness exaggerated, even though in the original comics he looks Ugly Cute and is usually regarded by other characters as more attractive than Asterix.
Even the regular stitchpunk versions of the characters get this treatment.
Expect Darth Maul to be more handsome and muscular in any fan art.
The Harry Potter fandom, of course. Snape was originally supposed to be rather unattractive, with greasy hair, pale skin, and a long, hooked nose. You would think this would have put more people off, but no. Of course, many of these people were simply finding the actor who portrays him in the movie to be rather attractive.
Lucius Malfoy is another example. He's not unattractive in the books, but that's because he's given virtually no description at all. Put Jason Isaacs in a blond wig and fancy wizard clothes? Now you're talking.
And Lucius' son, played by Tom Felton, isn't too shabby himself.
This actually came up in one of the interviews with J.K. Rowling, once — she mentioned not quite understanding Draco's leather-pantsed status, pointing out that she never said he was attractive in the actual books.
Hermione, however, is by far and large the best-known example of this trope; the fans who use her as a Possession Sue will have her undergo a makeover, making her bushy hair slick, and having her grow "curves in all the right places" over summer vacation, to get all the boys lusting after her. Saturday Night Live even ran a skit about it.
Also, Voldemort's mother Merope Gaunt. In canon she's described as very ugly, with both her eyes pointing in the opposite direction. Fanart of her tends to make her very cute and Moe.
Crowley is almost always depicted as suavely handsome in the Good Omens fandom, which does make sense. Aziraphale was also commonly portrayed as angelically beautiful in early GO fandom, but more recent fanworks seem to be moving away from this trope in regards to him, going for a more plausibly middle-aged, plump, and/or bookish appearance. Some of them do go the Nerds Are Sexy route, but it's still a far cry from pretty!Aziraphale.
Although he isn't a tremendously popular character, Reek aka Theon Greyjoy from A Song of Ice and Fire often gets this treatment in fanart. While in canon he is described as white-haired, emaciated, looks about twenty years older than he is, and just generally pitiful to look at, fanart tends to tone this down (most usually the premature aging). This tends not to be as extreme as other examples though, as his pitiful condition seems to be the source of his fans.
Raistlin in Dragonlance (described as bone-thin, weak and all-around unattractive) is prone to this, being depicted as a white haired Bishōnen in fanart, but then again he's depicted as quite handsome-looking in some official art so this trope is understandable.
Oh, the things the LazyTown fandom does to Sportacus and Robbie Rotten... The extreme athleticism of the former and... suspicious bulge of the latter may be bordering on Parent Service. Either way, the Periphery Demographic has made some... interesting things of both of them. And, from an entirely different quarter, Stephanie. She's quite often subject to She Is All Grown Up or simply artists forgetting how old she's supposed to be (namely, ten-ish.)
A big point of appeal for the character of The Doctor in Doctor Who is that, while all of the actors to have played him so far are rather attractive, it's usually expressed through force of personality rather than looks, with even the most conventionally handsome, Mr. Fanservice ones like Paul McGann or David Tennant being a bit funny-looking rather than generic Hunks or Pretty Boys. This is difficult to tell in fanart, however - it's not uncommon to see some shipper drawing Fourth Doctor/Sarah Jane fanart which does away with Tom Baker's Quirky Curls, big and oddly-shaped nose and Creepy Blue Eyes, even though they are the most fondly appreciated physical features of the character. On the other hand, a lot of other artists exaggerate the physical quirks of the Doctors, though these ones are usually not doing it for shippy reasons.
The Vocaloid Ryuto (Gachapoid) tends to lose his buck teeth in fan art.
Even Miku, the legendary ambassador of all things moe, receives this. Never mind that they have Meiko, Luka and Haku, some fans will draw Miku with huge breasts.
BIONICLE, features mostly robotic but still humanoid characters. It goes to reason that fan artists will try to humanize them even more, or smooth out their metal parts to create a sleeker appearance. For a more direct example, compare Toa Gali as a set◊ and in fan art◊.
The Tau are usually drawn with Mammary Glands and an average bust size of E. In Canon the only difference between the genders that humans can perceive is that females have a Y shaped mark between their eyes while Males have a I shaped one.
Tyrannids are drawn as Humanoid Aliens. The only features that make them look like Tyrannids is a bunch of Chitin plates and claws. If you didn't know better you would think it's a human female in a costume.
Necron pariahs are created from humans but they have a completely robotic appearance. The way fan artists draw them makes them look like a Sex Bot.
Imperial Guardswoman tend to be drawn in a much more Stripperific clothing than males even though in Canon their armor is much the same as any Guardsman.
Farseer Macha's entire character was pretty much re-purposed into a Ms. Fanservice by the fandom. Her only official appearance was in the Dawn of War PC game, and she was covered from head to toe in armor and robes (including her face), but that didn't stop fans from obsessing over her and creating lots of lewd fanfiction and art involving her.
Unsurprisingly, Samus Aran has been getting a triple serving of it ever since the introduction of the Zero Suit. See for yourself: original artwork,◊ Brawl,◊ fan art. Look on the bright side: at least it doesn't make her look too different from her canon portrayal. Although it can conflict with her status as an Action Girl.
Mario, Luigi, and even Wario and Waluigi have had this happen to them.
Mario is a short, plump, round-faced guy with a big nose and solid mustache. When the trope is applied, you get the most average looking anime character in existence that happens to be wearing a red cap with overalls, that used a pencil to scribble on their upper lip. Oddly enough he'll always stand taller than Peach, even though not a single video game has portrayed him taller than the princess. Luigi is the same, just taller. Wario replaces all fat with muscles, maybe a slightly thicker mustache, and Waluigi is just a sly, tall, lanky version of Mario.
Both Peach and Daisy also get this a lot, as well as Rosalina. Peach goes from a slightly ditzy lady with plain expressions, to a total sex goddess. In the main Mario games, there's absolutely nothing overly sexual about her character other than that she's pretty. However, when Super Smash Bros Melee included her into the roster, with its more-detailed art style, Peach's fanservice potential rose up considerably. That series has her making coy poses, attacking with her hips and butt, and showed off her legs. The fans amp up the boobs and tend to fetishize the princesses' dresses, if not putting them in other Stripperific outfits.
Dark Link, a mindless, faceless living shadow is almost always drawn as a white haired pretty boy, often with peach skin and normal eyes in reference to Shadow Link from the The Legend of Zelda: Four Swords Adventures manga.
The only time Dark Link has had a face (along with a regular Master Sword) is in Super Smash Bros. Brawl and even that's due to Palette Swap issues. When you fight him in an event match, Dark Link is his normal faceless self.
A good bit of Midna fanart (the ones by Japanese fan artist Dogear, in particular) up her from amply-curved to extremely amply-curved. Though there's just as much that goes the opposite extreme and makes her sort of a reverse-Pettanko...
Navi. In the game, she's a floating ball of light with wings; in the fanart she's sort of an angel with fairy wings & a skimpy costume/wearing nothing. This goes for other fairies in the series that have the same design, but of course Navi is the most well-known.
Fi also gets this. Despite her design being fairly androgynous, plenty of artists give her a more noticeably feminine figure. This doesn't always include giving her noticeable breasts, however, and some fan artists are willing to leave her with her slim frame, usually only emphasizing her legs or hips.
And when all the character art got redrawn for the PlayStation release, he was much better looking than his equip screen portrait - less like Orlok and more like Vegeta.
He always had Vegeta's face, except paler. And Piccolo's body structure (tall and muscular). Akira Toriyama tends to reuse character designs, after all.
Izanami, the True Final Boss of Persona 4, looks only slightly attractive in her non-human yet non-boss form. Then certain fanart transformed her rather gorgeous versions of herself.
Before it was made more apparent in Golden and the anime, Naoto tended to be shown with boobs bordering on silly in size in fanart, all due to a couple of relatively minor scenes where she gets flustered by the other girls pointing out her size compared to theirs. Rise also gets this treatment at times, even though she herself points out her modest size compared to her Shadow, who showed up in the previous night's Midnight Channel viewing.
Not entirely averted in the Star Control fandom, where female fans sometimes draw aliens in human bodies without making them sexy. Even Human!ZEX isn't a Bishounen... but the recent posts in the Livejournal community do tend to skew toward alternate-universe Hurt Comfort Fanart.
Giving the VUX flexible necks, longer head-tentacles, and a smaller eye goes a long way◊ towards making them more appealing while still being, well, VUX. Though that may have been artistic license and a need for a visible reaction to get the joke across in this case.
Even fans who aren't really trying to pretty them up generally draw them younger than the character models appear to be, i.e. as young adults or sometimes teenagers; the Special Infected may be Younger than They Lookas a side effect of the virus (this is a common fan theory), but they still look like they're at least in their thirties in-game. Again, this happens the most with the Witch and the Hunter. Mind you, getting close enough to take a good look at their faces in-game (as opposed to in Garry's Mod or looking at a reference picture) is difficult and inadvisable, so this is forgivable.
Also, since a lot of their grossness comes from icky details, simplifying an Infected's appearance will obviously make them cuter as well.
This (NSFW). While the Witch can be considerably cuter with removal of some of her infection details, the one with the pigtails is suppose to be the jaw-molten Spitter.
Characters from Ōkami find themselves subject to this quite a bit. Perpetuallymasked Oki is invariably assumed to be Bishōnen under the disguise and tiny, Napoleon Issun usually ends up as a cute boy. The most notable example has to be the protagonist, Amaterasu. In the game, she's a nonanthropomorphic wolf. In fanart... well, maybeyoushouldjustseeforyourself. (It is true that the wolf isn't Amaterasu's original form and that one character sees her as an attractive young woman, but the player doesn't get to see this, so it still counts as an example.)
Amaterasu has gotten to the point where people seriously want Amaterasu to appear in Marvel vs. Capcom 3 in her human goddess form... even though it doesn't exist in her game.
Final Fantasy VI's fans tend to have the Bishōnen Edgar has almost the same amount of muscle as his hunk younger twin, Sabin. There are also some fanarts that age up Relm and Gau.
Fans also tend to buff up Vaan and Balthier(both whom are reasonably muscular) to be more the size of Basch. Reverse Case also occurs.
A few Pokémon characters get this. For example, the game Silver has a large fan base that tend to make him more Tsundere and less of a Jerkass. His original design gets cleaned up, looking less harsh, but his remake one is already pretty Bishōnen.
Red is an interesting case. While he isn't ugly per say he is more or less quite young. Even at age fourteen in the original games, he is still a rather scrawny looking kid with unruly hair. Fans however ignore this, making him Bishōnen and combing his hair down. Even for the remakes, fans use this design, despite the fact he has a new design which is slick and Bishōnen at that age.
Pokemon Special's Red actually resembles fan!Red more closely. There are also some 4komas that give Red straighter hair.
Fan arts of feminine-looking Pokémon (Gardevoir and Lopunny especially) are usually drawn more attractively.
Mewtwo is quite popular among Furries.
Cynthia often gets a few cup sizes larger than she is in canon.
May is often drawn with breasts several cups larger than her canon one, which can't be bigger than a B and isn't very noticeable. This is partially thanks to the anime, where her breast size was very inconsistent until her reappearance in Sinnoh.
Fanart for Ghetsis freakin' Harmonia tends to ditch his robes and lessen some of his harsher features, like his missing arm. Often he ends up rivaling N for prettiness.
Granted, Splash Woman is cute and has a pretty good waistline for a mermaid, but most fanart of her (and there's a lot) puts her in all manner of seductive poses, give her a larger bust, and generally make her out to be some sort of goddess. Probably uncoincidentally, she's the first (and so, far, the only) enemy female Robot Master, and is also seen posing for a photo in Mega Man 9's ending.
Touhou, due to being about ninety percent fandom and possessing official character art that... leaves much to be desired. Oddly enough the majority of drawings are fairly true to the characters (young girls are drawn like young girls, older women are drawn like older women etcetera), though more bizarre interpretations are certainly not absent.
Touhou is a unique variation of this trope. Characters are not drawn more attractive merely to be more attractive, ZUN is simply a terrible drawer and therefore fans improve on them in an effort to portray their "actual" appearance. It helps that there are official examples, significant improvements to characters' apperances present in any official work in which ZUN did not draw them, most notably in the fighting Gaiden Games and guidebooks like Perfect Memento in Strict Sense and Symposium of Post-Mysticism. As a result, the insane amount of fanarts Touhou receive makes it a franchise where its popularity thrives on Self-Fanservice.
The non-human traits of many youkai characters tend to be interpreted by fanartists in whatever way will maximize their attractiveness.
The fanbase for Team Fortress 2 is full of this trope. Don't be surprised to find fanart of all the classes depicted as young and sexy (though in the Scout's case, he gets more of the latter due to being the youngest of the bunch). Yes, even the Heavy. As a fangirl favorite, Medic is especially prone to this. And let's not get started on the female versions of most characters.
The Pyro gets this in spades since no one knows what his true face looks like (or even if he's either male or female). Expect to find fanart of unmasked Pyro as either a pretty boy or an attractive girl.
An absolutely ridiculous example of this is the fanart that Giegue from MOTHER 1 often gets (Usually from the aformentioned Porky-Loving artists). His sprite in the game, while lacking detail due to it being an 8 bit game, clearly does NOT have boobs. He is referred to in game as male yet for some reason is drawn often with boobs in fanart. If that wasn't messed up enough, he is an alien. And mammaries are a trait of mammals, which are found on ''Earth.'' He does seem to have a slender, if feminine like body, but that's besides the point. In EarthBound, the sequel, there is a person who says that Giygas might be female. That's still besides the point. Even if he is indeed female, he, or she, is an alien.
Duster also gets this treatment quite a lot.
Juhani and Mical (aka "The Disciple") from Knights of the Old Republic have had their character models tweaked by fanart and mods to make them look more attractive.
On the fangirl end of the scale, there's Double H. Although he has a dorky, appealing personality, his character design isn't especially attractive. Fanart either tends to soften his extremer traits, making him more conventionally handsome, or ignore them completely to make him cute and adorable.
A divisive example of this is fan-made mods for games like Skyrim and the Dragon Age series, where in-game modifications to "beautify" characters like Aela the Huntress and Aveline exist, though male characters aren't immune either. Far more controversial are the mods that give Dragon Age's Isabella lighter skin, especially if they're explicitly stated by the mod's creator to be self-fanservice.
A lot of the more human characters from Yume Nikki get this treatment. Take Shitai-san, who is a dead guy in game. Most fanart depicts him as a Bishounen.
Pick any character from the Harvest Moon series who is the main farmer, both male and female, and the bachlor/bachlorette of their choice. Even the Shrinking VioletMeganekkos are usually drawn at a Bishojo level compared the chibi-like character designs (until A Tale of Two Towns and A New Beginning.
The humanoid women from Skylanders get hit by this a lot.
A weird example with Sarah Kerrigan from StarCraft. She starts out as a fairly sexy human character, who over the course of the series gets infested by the Zerg and turned into the Queen of Blades. While her infested form still looks sexy, the official look is more of a creepy kind of sexy, with major Uncanny Valley and Squick involved. Yet, a lot of fans ignore parts of those factors and made her look more sexy in fanarts. Ironically, StarCraft II has her gradually getting a relooking over the course of the game when she goes through a (sort of) Heel-Face Turn, leading her to get a slightly less disturbing, more beautiful look that would fit her portrayal in said fanarts.
Inverted in Nancy Drew: The Shattered Medallion. When previously-Unseen CharacterSonny Joon, a quirky slacker, was revealed to be attractive on the box art, some fans complained that he wasn't the pimpled geek they had imagined him to be.
Given the cartoonish style of Homestuck, there's a lot of room for interpretation as to how attractive the characters are, but the Trolls are generally drawn in fanart as pretty good-looking for grey-skinned, horned aliens.
It's actually inverted with the Exiles and other characters from Prospit/Derse, who are often drawn with carapaces, claws, and other details that are left out of the more stylistic art of the main series.
Snowman is just as often drawn with human features and texture resembling skin. Fanart including Spades Slick with her will render him similarly.
The scrawny, wheel-chair bound Tavros is depicted as being The Big Guy shockingly often.
The Troll Ancestors are often drawn with much smaller horns than they canonically have, but this is understandable since the horns can get insanely large. This can get less understandable for the Troll Kids' horns, however, which have obvious but simple designs but get reinterpeted quite often to suit the artist's desires. Tavros, despite having the largest horns by far, will often get drawn with smaller, sometimes thin horns for the sake of convenience.
Karkat, too. Fanon depictions of him often do away with the eye bags and sharp teeth, but the messy haircut tends to stay.
Whether cherub fanartists will play straight, avert, or invert this trope is often a crapshoot. On the whole, it seems that if a piece of art is supposed to be light-hearted, fluffy, or shippy, you can expect them to be drawn in a cutesy, Super-Deformed sort of style with their grotesque features minimized or stylized for maximum cutesiness. If it's supposed to be a more serious picture, depictions run the gamut from what they basically look like in canon to... well... (Spoilers here, obviously!)◊
The Striders tend to get this treatment by fanartists very often.
The elf Vaarsuvius is perfectly androgynous and, by implication, not necessarily attractive to human eyes. But give the character the Woobie treatment and make their modestly-lengthed (purple) hair grow to long, wild and luxurious (purple) lengths as a side effect of a magic spell, and what you get is fanart and fanfic that accentuate the character's ethereal and feyish frailty, their delicate appearance, etc. And don't forget the pointed ear fetishism.
It becomes apparent in Vaarsuvius fanart that his/her androgeny is difficult to translate into more detailed (i.e. not stick figure) art styles. So usually you get feminine features on an ambiguously feminine body.
Jerk City HD features all sorts of interesting interpretations of the Jerk City characters, but soon you'll notice a lot of the contributors like to draw Spigot the beatnik (the only obviously human character in the cast) very attractive.
Strong Sad: Uh, Strong Bad? Were you just first-basin' it with that piece of loose-leaf?!
On that matter, it looks like turning Strong Sad into a hoodie-wearing Emo kid has become increasingly popular. A rather thin one too.
Several humanizations of Homestar have him as a Pretty Boy with white hair.
While nobody is denying that some of the people of That Guy with the Glasses are attractive anyway, fangirls turning the boys into skinny bishies and fanboys making the girls thinner and bustier than they are in real life are both popular.
Taken to ridiculous levels in the Hate Fic/WankFic "Beer Enema". A lot of people think the Critic is attractive, yes, but there is no way you could think he has a "barbie doll face".
South Park gets rampant anime-style fanart of the boys as Bishonens. Sometimes, you realize that the skinny, bishonen guy in the red shirt is supposed to be Cartman.
Cartman is rarely drawn as obese as he seems on the show. He is usually just larger than the others. If he is drawn fatter by an artist, they tend to still give him some sort of attractive feature to compensate (anime-esque eyes being a popular one). On a different note most fanarts also don't draw Kyle with a jewfro (instead making it into a vaguely curly or straight hairstyle), give Kenny longer hair, and give Butters more hair.
Aged-up versions of Bebe expand on her early breast development from one episode and give her the largest bust size of all the girls, sometimes bordering on Gag Boobs. Her vague frizzy/curly style is either drawn as straight, curly, or super-long wavy flowing hair.
Even Terrance and Phillip get drawn as attractive men, when it's hard to decipher their looks within the cartoony split-head style used for the Canadian characters.
In the fandom's defence, the artstyle means that there's not much room for showing how attractive the characters are, so there's a lot left up for the imagination. Ike's teacher is the closest character to visible attractiveness, and even that was done for an aesop.
Zim is not the only recipient of Self Fanservice in the series. Far from it.
Did you know female Irkens are not actually supposed to have boobs? No, really, go check the show. See how flat Tak's chest is? Yeah, it's quite different than the fanart, isn't it?
Canon Dib is a scrawny little 11-year-old boy with glasses and a big head. Fanon depictions add 5-7 years to his age and make him Tall, Dark and Handsome instead while still retaining his trademark Anime Hair.
In Metalocalypse, one minor, self-conscious use of this trope in the fandom involves giving Murderface a more flattering haircut than the triangular 'do he has in the show.
Nathan also tends to lose his slight paunch and gets a sixpack instead.
In Kim Possible fanfics, Ron, who is honestly pretty goofy looking, is usually amped up considerably in the looks department... to the point that some authors both hang a lampshade on it and apologize to the reader.
There's a lot of Meg Griffin fanart on Deviantart, and just about all of it makes her look way more attractive than she is on the show. She's not as hideous as the show repeatedly claims, but she's not that pretty either. In this case, some of those pieces are as much a Take That as just Fanservice.
Despite what her trademark yellow jumpsuit may make one think, Teenage Mutant Ninja Turtles April O'Neil would very rarely display cleavage while wearing it—even in cases when she realistically would. Of course this attribute is commonly made prominent in fan art.
For that matter, the turtles themselves are outright portrayed as Bishōnen in many pieces of fanart.
Since the show began there has been lots of artwork of the main characters as humans, and said drawings frequently make them look sexy. Hasbro eventually released a humanized spinoff movie that they initially claimed was inspired by the wealth of fan art, although it has not been particularly well received by humanized pony fans and artists.
On a related note, human!Fluttershy is generally depicted in fan art as being the most well endowed of the main group of friends. It has become a running joke in the fandom that this incarnation of Fluttershy constantly wears loose turtleneck sweaters to hide her large breasts (though more fanservice-laden art just has her end up being a Sweater Girl), even though her Pony counterpart rarely wears clothes in the show. This trend usually carries over into anthro Pony art, as well.
Many Furry Fandom artists have a tendency to turn the Diamond Dogs from ugly humanoid canine-esque beasts into fairly normal-looking anthropomorphic dogs.
On a side note, both Nemo and Gill get to keep their damaged arm (fin) in all fanart.
On the topic of Pixar, humanizations of Mike Wazowski draw him as a handsome, green-haired emo guy with hair covering one eye.
Superjail! gets this a lot. The Warden is rather average-looking in canon, but you wouldn't know that from the fan art! And let's not get started on the twins and Stingray...
Alice tends to have her facial hair and other masculine features cleaned up a little in certain artists' styles, along with her brawniness being reduced. Other artists keep her squarish jaw and muscles, yet remove the stubble.
Jared and his Distaff Counterpart Charise are both short and pudgy, though some drawings will depict them skinnier, increase their height, or in Charise's case, increase her bust size. Jared's head will also find itself reduced in size at times.
A Moe Anthropomorphism of Jailbot gained some popularity among Japanese and Korean fanartists, where the robotic killing-machine became a lanky teenage android boy with white hair. Jailbot 2.0 and Nova are occasionally humanized as well, with the former becoming a Tall, Dark and Snarky figure and Nova suddenly becoming a tall shapely woman with pink hair.
Usually subverted with the inmates, who keep their rougher appearances in fanart (except if you're Nicky or Gary).
Happens quite a lot with Codename: Kids Next Door fan-art, which is inevitable considering 90% of the cast is under 13 and the art style isn't exactly sexy. But that still doesn't explain drawing Numbuhs 3 and 5 with large breasts.
What fangirls do to Numbuh 2 (and to a lesser extent the other boys) must be seen to be believed.
Several characters from Regular Show receive this treatment in some fanarts. Sometimes even Pops, Skips, and Muscle Man.
Adventure Time fans have a habit of drawing Finn an a slim, cute boy with Eternally Pearly-White Teeth, with his flowing blonde hair, maybe a gap between two teeth or a missing one. In the show Finn is missing a good portion of his teeth and is on the pudgy side.
Characters from Sponge Bob Square Pants are this when mixed with Moe Anthropomorphism. Patrick will hardly be fat and may be replaced with muscle, Sandy will go from skinny to rather well endowed and you can hardly find any Squidward that isn't bishonen - and they also tend to give him a luscious full head of hair.
Wow. I guess he gets doors slammed in his face a lot. (Two Faces of Squidward Reference)
Fans also seem to forget that Mr. Krabs is quite possibly the oldest reccurring character, and make him look youthful and attractive.
Even thought they're the two MsFanservices of the show, Starfire and Raven falls under this trope. Starfire is most of the time depicted as the bustiestherione of the girls and often put into her costume from the comics. Raven gets many butt shots and goes up a few cup sizes, just search her on Deviantart.